The present invention relates to the general field of turbine engines having one or two optionally ducted fans, and more particularly to controlling the pitch of the fan blades in such turbine engines.
A preferred field of application of the invention lies with turbojets having contrarotating propellers, also known as “open rotors”, comprising two contrarotating propellers, placed downstream (in a “pusher” configuration) or upstream (in a “puller” configuration) of the gas generator. Nevertheless, the invention also applies to turboprops having one or more propulsive propellers.
In a turbojet with one or more propellers, it is known that the pitch (or orientation) of the blades constituting such propellers constitutes one of the parameters enabling the thrust of the turbojet to be managed, in particular by causing the propeller to operate always under the best possible conditions. Specifically, the speed of the propellers is also constant during all stages of flight, and it is the pitch of the propeller blades that serves to vary thrust. Thus, during a stage of cruising flight, it is desired to obtain the lowest possible power on the turbine shaft that is compatible with given traction at a given airplane speed, so as to obtain the best efficiency (i.e. the efficiency that serves to minimize fuel consumption and increase range). Conversely, on takeoff, the highest possible traction is sought in order to cause the airplane to accelerate and then take off.
Typically, each propeller has a plurality of fan blades, each of which comprises an aerodynamic profile (or “airfoil”) and a pivot. The pivots perform several functions: via rolling bearings, they serve to retain the fan blades in operation and to guide them in order to set their pitch.
Furthermore, for turbojet architectures of the “open rotor” type in the “puller” version, the turbojet itself constitutes an oil enclosure that needs to be made leaktight (or from which leaks need to be under control). Unfortunately, on known fan blade pivot systems, since the structure situated beneath the propeller is not an enclosure, this sealing function is not guaranteed.
Furthermore, with known pivot systems, it is necessary to open the casing situated upstream from the propeller in order to be able to disassemble the pivots and perform maintenance on the blades. Nevertheless, with an “open rotor” type turbojet in the “puller” version, such a maintenance operation is particularly troublesome since it is necessary to rebuild the entire oil enclosure installation, including its pressurization. In addition, the presence of two propellers, fitted with their respective rotary drive shafts and their respective blade pitch control systems, greatly restricts access to the pivots of the fan blades. Furthermore, when it is necessary to perform a maintenance operation on a single downstream fan blade airfoil, it becomes necessary to remove the engine, and the upstream propeller assembly needs to be dismantled together with its systems, and so do the control systems for the downstream propeller.
Document U.S. Pat. No. 8,057,184 discloses a fan blade pivot in which all of the parts forming the pivot are subjected to all of the (centrifugal and aerodynamic) forces, which means that those parts need to be overdimensioned in order to enable the pivot to withstand stresses in twisting, in compression, and in stretching.
Furthermore, the pivot generates a plurality of concurrent force paths, which is not desirable and makes the pivot difficult to dismantle.
Also known from Document FR 2 953 195 is a fan blade pivot in which the balls of the bottom bearing need to be positioned after the pivot has been mounted, thereby making the bearing more complex to replace. Furthermore, all of the parts of the pivot are subjected to all of the (centrifugal and aerodynamic) forces, which means that those parts need to be overdimensioned in order to enable the pivot to withstand stresses in twisting, compression, and stretching.